Every grand claim that was made about the internet a decade ago has proven to be true. It has changed the way we work, the way we communicate with friends and family, the way we shop. In business, it has made millionaires of those who understood its potential and bankrupted many of those who did not. The sceptics have been refuted.

Naturally, such a transformative force has profound political implications. Unlike any medium before it, the internet puts the ability to publish information directly into the hands of ordinary people. It is an engine that liberates individual expression. It can be a powerful tool to spread democracy. As such, it is feared by repressive regimes. States that cannot tolerate dissenting voices have previously found it relatively easy to stifle them. Presses can be confiscated and radio signals jammed. But the decentralised nature of the internet - the way it routes information around the world with no regard for national borders - makes it difficult to censor. That has not stopped authoritarian regimes from trying. Citizens of countries such as China, Iran, Vietnam and Syria have been targeted - sometimes jailed - for posting opinions online.

That is why today, The Observer joins forces with Amnesty International to launch Irrepressible.info, a campaign to uphold free speech in the digital age.

Amnesty has a long and proud tradition of defending those who are silenced by the unjust exercise of state power. But one thing that makes this new campaign different is that it calls also on the private companies that provide the bulk of internet services to take some responsibility for what happens to dissidents. Digital giants such as Yahoo, Google and Microsoft stand accused of working in complicity with authoritarian regimes, customising their content at the behest of state censors.

In their defence, they say they are simply doing what all businesses do by obeying the laws of the land in which they operate. That is disingenuous. These companies have come from nowhere in a very short time to dominate a global medium. They do not own the internet and yet, de facto, they run it. They must accept that they have obligations to the wider online community as well as to shareholders and the bottom line.

It is in their long-term interests to do so. The immense power of multinational corporations, whether exercised benignly or otherwise, is part of a wider erosion of nation state sovereignty. Democratic governments that are regularly held accountable at the ballot box find themselves grappling with forces beyond their jurisdiction - international terrorism, climate change, organised crime, migration - and punished when they fail to deliver. There is a new interconnectedness to global issues that demands co-ordinated global action. The alternative is a retreat into protective nationalism.

Last week, Tony Blair made this interconnectedness the theme of a speech during his trip to Washington. He called for the strengthening of international institutions in general and reform of the UN in particular, expanding the Security Council and strengthening the job of the Secretary General. It is to Mr Blair's credit that he tried to bounce the US back into constructive engagement with the rest of the world on matters of foreign policy. But outside Washington, the Prime Minister's authority is weak. His ability to set the agenda for discussion of a new world order is undermined by his role in the bitter diplomatic feud over Iraq. That is a shame, since his prescriptions for the UN happen to be broadly the right ones.

But the missing element from Mr Blair's speech is the one identified by Amnesty International - the business giants who reach so widely and deeply into the lives of millions of people. No discussion of the challenges facing the world is complete without consideration of the companies that wield global power but see themselves accountable only to their shareholders. They are the big winners from globalisation and must face up to their responsibilities towards the losers, those who are excluded and, in repressive states, silenced.